Helicopter carried magnetometer assembly compensated for the parasitic field of the helicopter

ABSTRACT

A magnetometer assembly for detection of ferromagnetic targets carried by a helicopter is compensated for the parasitic magnetic field generated by the helicopter. First and second magnetometer heads are carried by the helicopter and located at points aligned with the axis of the main rotor. An electric control signal representative of the difference of the total magnetic fields at the first and second heads is obtained and is used to compensate the parasitic component due to the helicopter in the magnetic field at one of the points. The distance between the heads and the rotor is large as compared to the variations of the position of the magnetic center of gravity of the helicopter and as compared to the distance between the heads.

United States Patent Salvi 451 May 16, 1972 [73] Assignee:

Antoine Salvi, F ontaine, France Commissariat A LEnergie Atomique,Paris, France 22 Filed: Sept. 17, 1970 21 Appl.No.: 73,106

[52] U.S.Cl.

3,123,766 '3/1964 Ruddock et al ..324/4 DE TEC TING HEAD 7/ 1966 4/1969Salvi et a1 Ruddock et a1 ..324/8 ..324/43 R Primary Examiner-Rudolph V.Rolinec Assistant Examiner-R. .l. Corcoran Att0rneyCameron, Kerkam 8LSutton [5 7] ABSTRACT A magnetometer assembly for detection offerromagnetic targets carried by a helicopter is compensated, for theparasitic magnetic field generated by the helicopter. First and secondmagnetometer heads are carried by the helicopter and located at pointsaligned with the axis of the main rotor. An electric control signalrepresentative of the difference of the total magnetic fields at thefirst and second heads is obtained and is used to compensate theparasitic component due to the helicopter in the magnetic field at oneof the points. The distance between the heads and the rotor is large ascompared to the variations of the position of the magnetic center ofgravity of the helicopter and as compared to the distance between theheads.

9 Claims, 2 Drawing Figures DIFFERENCE @PLIFIER j Patented May 16, 19723,663,953

42 PHASEME7ER\ {AMPLIFIER F e e 3 E I DIFFERENCE FREQUENCY QMPLIF/ERMETER 7 e DETECTING I HEAD COMPENSATED FOR THE PARASITIC FIELD OF THEHELICOPTER The invention relates to compensation of the parasiticmagnetic fields associated with a helicopter transporting ahighprecision magnetometer and has for an object to eliminatedisturbances in the measurement of the earths magnetic field resultingfrom the parasitic magnetic fields due to the helicopter (in particular,the permanent fields and the fields induced by the ferromagneticmasses).

Various methods and devices for compensating parasitic magnetic fieldsassociated with aircraft carrying magnetometers already exist. Accordingto a prior art method the difference between the values of the magneticfield at two points for which the external magnetic field intensity issubstantially the same (whereas the intensitiesof the parisitic magneticfields due to the aircraft are different) is measured and processed toproduce a control signal proportional to this difference and thereforeto the parasitic magnetic fields. Among these methods, that described inU.S. Pat. No. 3,441,841 uses the control signal, in the form of anelectric DC variable current, to produce a compensating magnetic fieldwhose direction is opposite to that of the parasitic magnetic field, andwhich tends to cancel the said difference. I

In another prior art method, which has been described in pending U.S.Pat. application Ser. No. 866,501 assigned to the assignee of thepresent invention, a correction signal is introduced by means of anelectronic circuit into the output of the apparatus measuring themagnetic field at one of the points. This second method has over thefirst the advantage of less background noise, due to the absence ofcompensating coils permanently carrying electric current.

When the methods just described are applied to an aircraft, the fielddifference between two points aligned along the axis of roll of theaircraft and determined by two magnetometer heads provided in a tailextension (tail-boom) is measured, as is the absolute value for thefield at one of the points. This arrangement is difficult to apply to asingle-rotor helicopter because of the tail rotor; also, pilots do notappreciate having such an extension projecting in front of the cockpit,since it reduces visibility. So far, therefore, magnetometer prospectingwith helicopters has been carried out by placing the magnetometer in apod trailing on a non-magnetic cable between 50 and 100 m long. Thissolution has many disadvantages. In particular, it requires a winch onboard the helicopter and involves complex and somewhat dangerousmaneuvering when the cable is unwound after take-off and wound in beforelandmg. I

Magnetic research carried out on helicopters with a single main rotorhas shown that the chief magnetic anomaly is due to the rotor shaft, tothe extent that at a few meters from the helicopter the magnetic massesof the latter are relatively exactly comparable to a dipole coaxial withthe shaft of the main rotor. This property can probably be attributed tothe fact that the shaft, made from very hard steel, really acts as amagnet, and the effect of this magnet masks that of the remainder of thehelicopter from the magnetic point of view.

An object of the invention is to provide a method of compensatingparasitic magnetic fields associated with a helicopter sufficiently wellfor a high-precision magnetometer to be mounted on the helicopter andgive results undisturbed by parasitic fields (permanent fields andfields induced by the ferromagnetic masses of the helicopter).

To this end, the invention provides a method in which the total magneticfield difference between two points connected to the helicopter andseparated by a distance which is small compared with their distance fromthe barycenter of the magnetic masses of the helicopter, thelatterdistance being large compared with the shifts of this barycenter duringmaneuvering and changes in the loading of the helicopter, a controlmagnitude is produced which is proportional to the said difference, andthe parasitic component due to the helicopter in the magnetic fielddetermined at one of the said points is compensated using the controlmagnitude, characterized particularly in that the said points arealigned on an axis which coincides with that of the rotor shaft. inpractice, this method will lead to the use of a device having twomagnetometer heads situated in a tube of non-magnetic material fixedbeneath the helicopter and in an extension of the axis of the rotorshaft, that is to say, in a substantially vertical direction.

It should be noted that the invention is applicable to nuclear magneticresonance magnetometers having either compensation by means of coils,like those described in the above-mentioned U.S. patent specification,or electronic compensation of the type described in pending U.S. Pat.application Ser. No. 866,501, and to magnetometers of any othertype-Also, on helicopters with a digital computer on board, itis'possible to digitize the output signals representing the totalmagnetic field at one of the points and the difference between themagnetic fields at the two points, by means of analog-to-digitalconverters (such as a digital frequency meter, where measurement of thefield amounts to a frequency measurement, or a digital voltmeter with alarge enough number of decades), and to supply these data to thecomputer, which operates-in real time. This solution has the additionaladvantage, in the case of magnetometers subjected to other disturbingactions (such as the gyromagnetic effect which produces an error inmeasurements carried out with nuclear magnetic resonance magnetometers),of being able to apply the corresponding correction factor, also usingsignals from the rate gyros included in the navigational equipment ofthe helicopter.

In general, the method and device defined above can be used with anyhigh-precision magnetometer, for example optical pumping magnetometersor nuclear magnetic resonance magnetometers such as that described inthe above-mentioned U.S. patent specification. The method and device aresuitable whether the magnetometer is intended to provide magneticmapping of the terrain flown over (and therefore to give a preciseindication of the absolute value for the magnetic field) or to detectdisturbances in the earths magnetic field. In the latter case, in whichthe magnetometer is associated with means for eliminating the continuousor very slowly varying component of the field, the invention is in factparticularly useful, since highly sensitive detection is required ratherthan precise measurement of the field variations in space. In this caseit is desirable to'make the distance between the points at which thefield difference is measured such that, when there are appreciablemagnetic anomalies in the zones fiown over, it is no longer possibleto-consider that the external magnetic field intensity is exactly thesame for the two points.

The magnetic anomaly due to the earth will in fact provide, between twopoints aligned vertically beneath the helicopter, a total field moduledifference opposite to that due to the helicopter. This oppositedifierence will produce a signal which will cause the measuring resultto have a value greater than the actual value for the disturbance, and,in other words, will amplify variations and facilitate detection ofthem. The device measuring the total field difference between the twopoints is incapable of determining the cause of this difference, and itapplies the correction factor to the measurement 1 whether thedifference is due to the helicopter carrying it or to externaldisturbances.

This arrangement has other, related advantages. For example, thevariation in the vertical gradient when passing over a magnetic anomaly,and therefore the amplification, will be at a maximum when the lines offorce of the earths field approach the horizontal, that is to say, inconditions for which the variation in the disturbance due to an anomalyis slowest.

The invention will be better understood from the following descriptionof a particular embodiment, given by way of example only and using anuclear magnetic resonance magnetometer and electronic compensation ofthe type described in U.S. Pat. application Ser. No. 866,501.

The description refers to the accompanying drawings, in

which FIG. 1 illustrates very diagrammatically a helicopter with avertical tube containing the two heads, situated at the points betweenwhich the field difference is measured; and

FIG. 2 is a basic diagram for the detecting and compensating circuit.

FIG. 1 is a very diagrammatic side elevation of a helicopter with acockpit 10, a fuselage of which the rear consists simply of a metallattice girder l2, and a turbine 14 for driving a main rotor 16 mountedon a shaft 18. The gyroscopic torque of the main rotor 16 is balanced bymeans of a tail propeller 20. Measurements carried out on helicopters ofthis type have shown that at a distance of a few meters the effect ofthe magnetic masses of the helicopter is comparable at least below thehelicopter, to the effect of a dipole 22, the direction of whosemagnetic moment M is identical to that of the axis of the shaft 18.

According to the invention, the helicopter is equipped with amagnetometer assembly comprising two detecting heads: a head situated ata distance x 8x from the dipole and hereinafter termed the compensatinghead T and a head situated at a distance x, hereinafter termed thedetecting head T,.

According to a first feature of the invention, the two heads T and T,are situated on a line which extends the axis of the rotor shaft 18 andtherefore the dipole 22. In other words, the two heads are both situatedin the first Gaussian principal position. Also, x is given a valuedistinctly greater than the length of thedipole to which the helicopteris comparable, and Ex is given a value distinctly less than x. Thevalues for the magnetic field at the points occupied by the heads T, andT can therefore be regarded as including two terms, one corresponding tothe earths magnetic field and the other to the disturbances due to thehelicopter, that is, the disturbances due to the dipole 22.

Using the same notation as above, the disturbing fields AH, and AHacting on the heads T, and T are respectively:

(Fax 1 M AVA-2U? The difference AHr AH, between the parasitic magneticfields at the points occupied by the heads T and T, will be equal to thedifference between the magnetic fields measured by the heads T and T,when the two heads are sufficiently close for the earths magnetic fieldto have the same value at the two points. With this hypothesis, it ispossible to deduce from AH AH, a control magnitude proportional to thedisturbance AH, caused by the helicopter at the head T,, and to use itto compensate the disturbance.

In practice, the heads T, and T, will be placed in a nonmagnetic tube 24beneath the helicopter. In particular, the tube may be made ofglassfiber-reinforced synthetic plastic material. The tube must, ofcourse, be retracted before landing. For this purpose it may be designedto swivel around a flange 26 attached to the bottom of the cockpit. Thetube can then be situated directly in the extension of the rotor shaftwhen in use. Alternatively, a telescopic tube may be used, but in thiscase the tube cannot be placed exactly along the axis. Its fixed upperportion, into which the extensible portions retract, will be offsetrelative to the rotor shaft and so positioned and directed that thelower portion containing the heads is substantially in line with theshaft. By way of example, the axis of a tube of this kind is indicatedby chainlines in FIG. 1.

On a single-rotor helicopter of the size of an Alouette [1,

for example, there could be three telescopic portions 2.50 m long, whichwould project about 7.50 m beneath the cockpit floor, and the distancebetween the two heads could be of the order of l in. More generally, thedistance x will be between 7 and 10 m and the distance 8x will bebetween 0.5 and l m (fix/x between H20 and 1/7).

As already indicated, the compensation based on the control magnitudedetermined by finding AH AH, can be carried out by various methods. Byway of example, FIG. 2 gives a basic circuit diagram for a device forcarrying out the method described in U.S. Pat. application Ser. No.866,501 and illustrated in FIG. 5 of this Application.

Head T, forms, with a loop amplifier 30 and associated components (notshown), a spin oscillator of the type described in U.S. Pat. No.3,249,856, and provides at its output 32 an alternating signal whosefrequency is directly prov portional to the intensity H of the totalmagnetic field at the point occupied by the detecting head T,. Thissignal, which is amplified in the loop amplifier 30, is applied to afrequency meter 34, which supplies at its output a voltage: e, k,. H,.

The compensating head T on the other hand, operates as a nuclear filter.Its input 35 is connected to the output from the amplifier 30, and thesignal obtained at its output 36 is out of phase with the input signal.This output signal is fed to an amplifier 38. The output voltages fromthe amplifiers 30, 38 are applied to the input of a phasemeter 40, whichprovides at its output a voltage proportional to the phase shift betweenits input voltages, at least in the case of slight phase shifts. Theoutput voltage e, of the phase meter is therefore linked to the fieldintensities H, and H at the points occupied by the heads T, and T, bythe relation:

Preferably, the frequency meter used is such that k ,=k.

If, also, the earth's field has the same value H at the points occupiedby the heads T, and T we have e =k AH AH,).

As a result, compensation is carried out by amplifying e, at a ratioequal to .r/38x in a variable-gain amplifier 42 and subtracting thevoltage e so obtained from the voltage e,. This operation is carried outin a difference amplifier 44, whose output provides a voltage eproportional to the field intensity H e=kH Obviously, the controlmagnitude could equally well be used to automatically control thecurrent flowing in compensating coils provided substantially at thebarycenter of the magnetic masses of the aircraft (at the center of thedipole 22), using the method described in U.S. Pat. No. 3,441,841, or intwo coils placed around each of the heads.

The preceding description assumes that the values for the earthsmagnetic field at the points occupied by the heads T, and T are equal.In fact, when the purpose of the magnetometer is to detect anomalies inthe earths field instead of carrying out absolute measurements of thisfield, it is preferable to move the heads further from the helicopterand to make the distance 8x such that this hypothesis is not completelyrespected, at least if the field is disturbed by local magneticanomalies. The advantages of this arrangement are obvious if onerealizes that the compensating device is incapable of distinguishing theorigin of the gradient between the two heads and applies a correctioncoefficient proportional to the field difference existing. As thealtitude increases, the undisturbed terrestrial field decreases by about1 gamma every 30 m at average latitudes, and its gradient is thereforeopposite to that due to the helicopter, but substantially constant. Bycontrast, a localized magnetic anomaly will, when flown over, produce arapid variation with time in a gradient of direction opposite to thatdue to the helicopter: the compensating device will tend not tocompensate it, but to amplify it.

This amplification can be worked out approximately by using asimplifying hypothesis, consisting in admitting that the helicopter,comparable to a vertical dipole M and equipped with two heads atdistances at and x- 8x from the dipole, passes at a height 2: abgye anobject which, also, is comparable to a vertical dipole M. This caseapproximates to that of a sub- 5 marine remaining in one hemisphere andmagnetized by influence in the earths magnetic field. The effect at adistance is in fact comparable to that of a horizontal dipole and avertical dipole M situated at the level of the conning tower.

The simplified calculation made above shows that we have approximately:

In these formulas, which apply if 8x x, AH, and AH, designate thedisturbances created by the helicopter at the heads.

To compensate the AH action of the helicopter at the detecting head, thecompensating system creates a control magnitude equal to AH, andobtained from AH, and AH which at first are assumed to be due solely tothe helicopter.

The order of magnitude of this correction can be estimated. At 7 mbeneath a single-rotor helicopter such as the Alouette ll, thefieldmodule difference between two heads 1 m apart is between 03y and 0.4which becomes a AH correction of the order of 0.77

As for the effects 611 and 5H of the vertical dipole M 5 formed by thesubmarine, they are linked by the equations:

The compensating circuit works out a correcting magnitude:

that is:

and the variation measurement provided by the amplifier 44 corresponds,in fact, not to a value 6H but to 81! (A110 that is, to 811, (l x/z). Inother words, the variation, that is, the signal, is amplified in a ratioof the order of l +x/z: If x= 8 m and z 150 m, the amplificationcoefircient is of the order of 0.5 percent. This is the case when ahelicopter patrolling at 70 m passes over a submarine at a depth of 90m.

However, a helicopter may fly at an altitude lower than that justmentioned, which is used by patrolling aircraft only for reasons ofsafety. If the helicopter is at 30 m and the submarine at a depth of m,the amplification is approximately 1.10 percent.

The existence of this coefficient is the more interesting 7 because thecoefficient decreases if the dipole is deeper,

reducing the incidence of background anomalies compared with theanomalies sought. This property is particularly useful above thecontinental shelf, where it is difi'rcult to distinguish the effects offerromagnetic masses on the surface from those of submerged masses.

I claim:

1. A method of compensating a magnetometer system carried by ahelicopter having a single main rotor for parasitic magnetic fieldsassociated with the helicopter, the steps of measuring the differencebetween the magnetic fields at two points connected to the helicopterand separated by a distance which is small compared with their distancefrom the barycenter of the magnetic masses of the helicopter, the latterdistance being large compared with the amplitude of the displacements ofsaid barycenter during maneuvering and changes in the loading of thehelicopter, deriving from said difference an electric control signalproportional to the said difference, and compensating the parasiticcomponent due to the helicopter in the magnetic field measured at one ofthe said points using the control signal, wherein said points arealigned on an axis which coincides with that of the rotor shaft.

2. A method as claimed in claim 1, wherein compensation is carried outby adjusting automatically and in proportion to the control signal a DCelectric current in'compensating coils situated substantially at thebarycenter of the magnetic masses of the helicopter.

3. A method as claimed in claim 1, wherein compensation is carried outby multiplying said control signal by a predetermined value representingthe ratio between the parasitic field at one of said points and saiddifference, and by subtracting the electric signal so obtained from anelectric signal representing the field at one of said points.

4. A magnetometer assembly for a helicopter having a single main rotorcarried by a shaft, said assembly being compensated for the parasiticmagnetic field generated by the helicopter, comprising a first and asecond magnetometer head carried by the helicopter, means for providinga signal representative of the value of the total magnetic field sensedby the first head, means for determining the difference between therespective values of the total magnetic field sensed by the first andsecond heads, means for generating a .control voltage in directproportion with said difierence, and

means responsive to said control voltage for modifying said signal,wherein the improvement consists in that the first head and the secondhead are located at points aligned with the axis of the shaft of saidrotor, the length between each of said points and the shaft being largeas compared to the variations of the position of the magnetic center ofgravity of the helicopter and as compared to the length between thefirst head and the second head.

5. An assembly according to claim 4, wherein the said heads are situatedin a tube of non-magnetic material fixed beneath the helicopter and inan extension of the axis of the rotor shaft.

6.A magnetometer assembly according to claim 5, wherein the tube ispivotally connected to the helicopter around a flange mounted on thefloor thereof.

7. A magnetometer assembly according to claim 5, wherein the tube istelescopic and the lower portions can be extended from a fixed upperportion offset relative to the rotor shaft and so positioned angularlythat the lower portion containing the heads is substantially in linewith the shaft.

8. A magnetometer assembly as claimed in claim 4, for detectinganomalies in the earths magnetic field, characterized in that thedistances x between the heads and the rotor shaft and 8 x between thetwo heads are such'that the values for the earths magnetic fielddisturbed by the anomalies are not equal at the level of the two heads.

9. A device as claimed in claim 8, wherein x is between 7 and 10 m and 6x is between 0.5 and l m.

1. A method of compensating a magnetometer system carried by a helicopter having a single main rotor for parasitic magnetic fields associated with the helicopter, the steps of measuring the difference between the magnetic fields at two points connected to the helicopter and separated by a distance which is small compared with their distance from the barycenter of the magnetic masses of the helicopter, the latter distance being large compared with the amplitude of the displacements of said barycenter during maneuvering and changes in the loading of the helicopter, deriving from said difference an electric control signal proportional to the said difference, and compensating the parasitic component due to the helicopter in the magnetic field measured at one of the said points using the control signal, wherein said points are aligned on an axis which coincides with that of the rotor shaft.
 2. A method as claimed in claim 1, wherein compensation is carried out by adjusting automatically and in proportion to the control signal a DC electric current in compensating coils situated substantially at the barycenter of the magnetic masses of the helicopter.
 3. A method as claimed in claim 1, wherein compensation is carried out by multiplying said control signal by a predetermined value representing the ratio between the parasitic field at one of said points and said difference, and by subtracting the electric signal so obtained from an electric signal representing the field at one of said points.
 4. A magnetometer assembly for a helicopter having a single main rotor carried by a shaft, said assembly being compensated for the parasitic magnetic field generated by the helicopter, comprising a first and a second magnetometer head carried by the helicopter, means for providing a signal representative of the value of the total magnetic field sensed by the first head, means for determining the difference between the respective values of the total magnetic field sensed by the first and second heads, means for generating a control voltage in direct proportion with said difference, and means responsive to said control voltage for modifying said signal, wherein the improvement consists in that the first head and the second head are located at points aligned with the axis of the shaft of said rotor, the length between each of said points and the shaft being large as compared to the variations of the position of the magnetic center of gravity of the helicopter and as compared to the length between the first head and the second head.
 5. An assembly according to claim 4, wherein the said heads are situated in a tube of non-magnetic material fiXed beneath the helicopter and in an extension of the axis of the rotor shaft. 6.A magnetometer assembly according to claim 5, wherein the tube is pivotally connected to the helicopter around a flange mounted on the floor thereof.
 7. A magnetometer assembly according to claim 5, wherein the tube is telescopic and the lower portions can be extended from a fixed upper portion offset relative to the rotor shaft and so positioned angularly that the lower portion containing the heads is substantially in line with the shaft.
 8. A magnetometer assembly as claimed in claim 4, for detecting anomalies in the earth''s magnetic field, characterized in that the distances x between the heads and the rotor shaft and delta x between the two heads are such that the values for the earth''s magnetic field disturbed by the anomalies are not equal at the level of the two heads.
 9. A device as claimed in claim 8, wherein x is between 7 and 10 m and delta x is between 0.5 and 1 m. 